public class kehou3_3b {
	public static void main(String args[]) {
		double sum = 0;
		int i = 1;
		do {
			sum=sum+1.0/fact(i);
			i++;
		}while(i<=20);
		System.out.println(sum);
	}
	public static int fact(int n) {
		if(n==0)
			return 1;
		else
			return n*fact(n-1);
	}
}
